While doctors are disappointed to find traces of HIV in the nearly 4-year-old child, that she remained HIV-free for two years is an encouraging sign for the experimental treatment.

A Mississippi child who was born with HIV but had remained free of the virus for more than two years after early treatment now has detectable levels of the virus, according to the researchers involved in the case.

The child, known as the "Mississippi baby," was born to a HIV-positive mother in 2010 and had been treated with antiretroviral drugs beginning in the first hours of life, and continuing for 18 months. But the doctors lost contact with the family, and the baby didn't receive medication.

The baby's case became well known when the child returned to the hospital after five months of being out of touch, and to the surprise of the doctors, showed no sign of the virus in tests.

The child remained HIV-free for the following two years, and the case spurred excitement in the medical and scientific community, and led to planning for a clinical trial to build upon the findings with funding from the National Institutes of Health.

But now, during a routine doctor's visit earlier this month, the nearly 4-year-old child was found to have traces of HIV in her blood.

"Certainly, this is a disappointing turn of events for this young child, the medical staff involved in the child's care, and the HIV/AIDS research community," Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, said on July 10. "Scientifically, this development reminds us that we still have much more to learn about the intricacies of HIV infection and where the virus hides in the body."

When the child was first found to be HIV-free, the doctors emphasized that the child could be considered "cured" of HIV if more time passed, and the HIV levels remained undetectable.

However, the last test results have shown detectable HIV levels in the child's. The doctors also found the child had a weakened immune system, and HIV antibodies are present, a sign that the virus is active in the body. The child was again started on antiretroviral therapy.

The new findings suggest that the treatment didn't completely eliminate all the virus from the body, the doctors said. But they also said they are still hopeful the treatment the child received at birth, which kept the baby HIV-free for two years, will give clues about HIV treatment.

"The fact that this child was able to remain off antiretroviral treatment for two years and maintain quiescent virus for that length of time is unprecedented,” said Dr. Deborah Persaud, one of the pediatric HIV experts involved in the case. "Typically, when treatment is stopped, HIV levels rebound within weeks, not years."

Next, the researchers need to understand why the treatment was successful in keeping the virus from rebounding for so long, and whether a period of sustained remission without receiving medication could be prolonged even further, Fauci said.